Modify *ASSERTMSG() so they are now used as variadic macros. The main goal
is to provide routines that do as KASSERT(9) says: append a message
to the panic format string when the assertion triggers, with optional
arguments.
Fix call sites to reflect the new definition.
Discussed on tech-kern@. See
http://mail-index.netbsd.org/tech-kern/2011/09/07/msg011427.html